Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention and/or to assess its effectiveness. IVUS imaging uses ultrasound echoes to create an image of the vessel of interest. The ultrasound waves pass easily through most tissues and blood, but they are partially reflected from discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module (PIM), processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the catheter is placed. The image cross-section in a typical IVUS scan is formed of a plurality of radial A-scan lines. Each A-scan line results from echoes collected from an ultrasound signal emitted by the catheter. The ultrasound signal is a pulse forming a beam propagating radially outwards from the intravascular device towards the blood vessel, which defines the longitudinal direction.
Conventional IVUS imaging methods produce images while a transducer in the catheter head moves through a blood vessel during a scan (e.g., during a pullback). The ultrasound signal is typically collected over a small angular sector of the cross-sectional image one at a time, rather than simultaneously around 360 degrees. Thus, radial A-scan lines are not necessarily co-planar as they are collected at different longitudinal positions as the catheter moves along the blood vessel. Typical IVUS images are displayed as if the A-scans comprising a 360° rotation about the longitudinal direction are co-planar even though they are not. The result is a discontinuity between the first (0°) and the last (360°) radial scans because of the different longitudinal positions between the first and last radial beams. This discontinuity is called a ‘seam’ and is typically arbitrarily placed either at the 3 o'clock or the 12 o'clock position in an IVUS image. The seam is especially visible if image was acquired from a very fast pullback, or around tissues with high motion. The seam between the 0° and the 360° scans not only distorts the image, but it displays non-coplanar data points as if collected at a plane orthogonal to the longitudinal direction of the blood vessel. The difference of longitudinal position between two consecutive radial A-scans, and between the points along an A-scan being not perpendicular to the longitudinal direction is sometimes neglected because it may be visually unnoticed for relatively small pullback speeds vz. In reality, collected data points correspond to a twisted band (or helical plane) bounded on the outside by a helix (or cork screw) and on the inside by a longitudinal axis extending along the blood vessel. Inaccurate imaging becomes more pronounced as catheter withdrawal speeds are increased, as the difference in longitudinal distance lengthens between the beginning (0°) and end (360°) of the IVUS image.
Accordingly, there remains a need for methods of scan conversion and associated systems and devices that correct the seams resulting from non-coplanar radial A-scan lines in IVUS imaging.